Japanese Patent registration No. 3299829 discloses a variable directional capacitor microphone including two capacitor elements each of which has a vibrating plate and a fixed electrode. The two capacitor elements are combined as to supply a part of sound pressure impressed to a vibrating plate of the one capacitor element (a rear capacitor element) to the back side of a vibrating plate of the other capacitor element (a front capacitor element). The structure will be described referring to FIGS. 3 and 4. FIG. 3 is a cross sectional view of the variable directional capacitor microphone which has been assembled and FIG. 4 is an exploded cross sectional view of that of FIG. 3.
The variable directional capacitor microphone includes a first and a second capacitor elements 10a, 10b. Each of the capacitor elements has the same structure so that the first capacitor element 10a will be explained hereinafter. Constitutional elements corresponding to the second capacitor element 10b are attached with the same reference numerals as that of constitutional elements corresponding to the first capacitor element, together with a reference symbol “b”.
The capacitor element 10a has a case 16a which is formed in a ring shape with electric insulating material. A vibrating plate supporting member 12a, a spacer ring 13a, a fixed electrode 14a and pedestal 15a are assembled in the case 16a in this order. A vibrating plate 11a is fixed and strained with a predetermined tension force on the vibrating plate supporting member 12a. 
The case 16a has an inner edge flange 161 latched with the circumference of the vibrating plate supporting member 12a and female screw threads 162 formed on an inner surface of a body of the case. The outer circumference of the pedestal 15a has male screw threads 151 screwed with the female screw threads 162. Therefore, the pedestal 15a is screwed to the case 16a so that the vibrating plate 11a and the fixed electrode 14a are faced together and are securely fixed through the spacer ring 13a. 
An electrode rode 121 is extracted from the vibrating plate supporting member 12a. FIG. 3 or 4 illustrates only one through hole 141, however, the fixed electrode 14a has a number of through holes 141. The pedestal 15a is formed in a saucer-shape with the circumference of the pedestal 15a protruding such that an air chamber having a predetermined air volume is formed between the pedestal 15a and the fixed electrode 14a. The center of the bottom of the pedestal 15a has a through hole 152. The both sides of the through hole 152 are covered with two sheets of acoustic resisting members 17a and 18b formed with nylon mesh or the like.
The first and the second capacitor elements 10a and 10b are combined through a connecting ring 22 having female screw threads 221 in the state the pedestals 15a and 15b are faced back-to-back and a gasket 21 formed in a ring shape is disposed between the both pedestals.
Male screw threads 151 of the pedestal 15a are screwed from one side of the connecting ring 22 and on the other hand, the male screw threads 151 of the pedestals 15b are screwed from the other side of the connecting ring 22. Then the first and the second capacitor elements 10a and 10b are combined with each other through the connecting ring 22.
A first chamber A1 is formed in the space between the fixed electrode 14a and the pedestal 15a of the first capacitor element 10a, a second air chamber A2 formed in the space between acoustic resisting members 17a and 17b, a third air chamber A3 formed in the space of the center porting of the gasket with the both sides of the spaced surrounded by the pedestals 15a and 15b, a fourth air chamber A4 formed between the acoustic resisting members 17b and 18b of the second capacitor element 10b, a fifth air chamber A5 formed between the fixed electrode 14b and the pedestal 15b. The back sides of the vibrating plates 14a and 14b acoustically communicate through the acoustic capacities connecting in a ladder-form of the five air chambers A1 to A5.
According to the prior art of the structure described above, each of the first and the second capacitor elements 10a and 10b can be operated respectively before the both elements are combined so that a pair of capacitor elements having similar technical performances are selected and combined to obtain a microphone having wholly stable characteristics.
The variable directional capacitor microphone of the prior art described above is required that the output of each of the capacitor elements 10a, 10b has a satisfactory cardioid directional characteristic in the state that each of the capacitor elements has been assembled.
Therefore, the capacitor elements 10a, 10b are combined, after each of the capacitor elements has been adjusted such that an acoustic resistance of each capacitor element has a predetermined value.
However, no expected characteristics may be obtained after the capacitor elements have been combined. In this case the acoustic resistance should be re-adjusted. In the above-described example of the prior art, the acoustic resistance cannot be adjusted in the state that the both capacitor elements have been left combined so that the connecting ring should be removed and the acoustic resistance should be re-adjusted after each of the capacitor elements has been decomposed. However, there is no guarantee that the adjustment of the acoustic resistance is completed only once.